Controlling the current

Resistance has been something a conductor happens to have. From here it becomes something you choose, in order to get the current you want.

Figure to be added

Figure 19.20 — a tap fitted to a pipe, controlling the flow of water through it.

A tap can be fitted to control a stream of water flowing through a pipe. What the tap does is obstruct the flow, and by obstructing it the water current is brought under control.

The same trick with a current

An electric current flowing through a conductor can be controlled in exactly the same way, and you already know what to do. Increasing the resistance of a circuit reduces the current flowing through it — and Ohm's law says why.

V = I R

Hold V constant and increase R, and I falls.

Figure to be added

Figure 19.21 — the electric current flowing through a conductor being controlled by a resistor placed in the circuit.

Resistors

Components manufactured with various resistance values, to be connected into a circuit in order to raise its resistance, are called resistors.

A resistor is a deliberate obstruction

Every wire in a circuit has some resistance, and most of the time that is a nuisance. A resistor is resistance put there on purpose, with a value chosen to give exactly the current the circuit needs.